A spacecraft in orbit is subject to various disturbance forces that affect its ability to maintain its station, i.e., desired orbit and position on the desired orbit. To counteract these forces, spacecraft are generally equipped with thrusters for station keeping maneuvers. Existing approaches to handle station keeping requirements use impulsive propulsion systems that are manually commanded from a ground control center.
In addition to orbital perturbations, spacecraft are disturbed by external torques that are generally absorbed by onboard momentum exchange devices, such as reaction wheels or control moment gyroscopes, allowing the spacecraft to maintain a desired orientation relative to the Earth or stars. To prevent saturation of the momentum exchange device and subsequent loss of the desired spacecraft attitude, the stored angular momentum is periodically unloaded via the onboard thrusters, which is also a manually commanded process from a ground control center.
The process of determining and commanding the onboard thrusters from a ground control center is manual, tedious and does not easily scale to the increasing number of spacecraft in particular orbits, e.g. geostationary orbit, and their tight station keeping windows as required, for example, for spacecraft co-location. Also, such a manual control results in an open-loop strategy, which is not able to automatically correct for errors introduced in the modeling or implementation of the desired station keeping and momentum management maneuvers, thus resulting in limited precision positioning and pointing of the spacecraft.
Generally station keeping and momentum unloading are achieved by a different set of thrusters, which is undesirable due to mass being a driving consideration in spacecraft design, and due to the increase in complexity and cost. Combined station keeping and momentum unloading problem using the same set of thrusters results in multiple objectives, and methods for coordinating such objectives in order to achieve them concurrently are challenging, see, e.g., method described in U.S. Pat. No. 8,282,043 that simplify the control by using maximum values available for torques and forces of the thrusters.
Furthermore, there are often restrictions on the placement of thrusters on a spacecraft so that antennas and solar panels may be deployed without the risk of thruster plume impingement. Restricting the placement of thrusters that are used for both station keeping and momentum unloading may mean that the thrusters would be unable to provide pure torques without also applying a net force on the satellite. Therefore, firing the thrusters may affect the spacecraft position and orientation (pose) as well as the stored momentum, creating a problem of concurrent station keeping, attitude control, and momentum management.